And, it found, aircraft could lose GPS altogether when flying at under 2,000 feet over metropolitan areas - such as when they're taking off or coming in to land.

The interference is taking place because Lightsquared's spectrum block is right next to that used by GPS receivers.

"The test data discussed today makes clear that there is substantial interference to GPS if LightSquared turns on high-powered terrestrial facilities in the spectrum next door to GPS," says Jim Kirkland, vice president and general counsel of Trimble, a founding member of the Coalition to Save Our GPS.

"The data confirm what the industry told the FCC before it granted the waiver, and also confirms that there is no viable technical fix. It's time for the FCC to stop squandering resources trying to find a solution to an unfixable problem. Instead, it should focus its efforts on finding spectrum that LightSquared can operate in - where LightSquared won't interfere with GPS."

The review suggests that there are things that can be done. LightSquared's service could be moved to a different frequency band, or reduced in size to remove the area of conflict. Another possibility, though fairly impractical, perhaps, would be to install filters on GPS receivers to reject or limit LightSquared transmissions.

A joint industry report is due to reach the FCC on June 15th; there will then be a public comment period before the FCC makes its final decision.